This invention relates to a method and apparatus for a systemic roof control of native roof strata in a mine opening wherein natural roof strata is utilized as a major structural component in the system to convert incompetent roof strata into an effective continuous beam which spans across a mine opening. More particularly, the present invention utilizes a prestressed tensile member connected at anchor sites horizontally remote from the exposed roof strata to impose reactive forces upon the roof strata as a distributed compressive stress while the tensile member converts to infintely variable resultant vertical force components which prestress the native roof strata in a manner to increase the friction between adjacent strata to impart shear resistance. The novel feature of the invention is that the prestressing forces are imparted uniformly into the strata without harmful stress gradients that contribute to shear fractures.
Usually, the problem in the personal safety for mine workers is that there is too little space to produce coal economically and at the same time to provide protection to mine workers. Mechanical roof supports that interfere with the productivity are not an acceptable solution to the problem.
Mine openings have been supported in the past by timbers, concrete, metallic structures and, more recently, by roof bolts. Experimental devices have been developed for supporting the entries of mine openings wherein these devices take the form of mobile roof supports that are hydraulically operated. Other suggested measures include the use of plastic adhesive to impregnate the roof strata, or using shotcrete or coating techniques for protecting roof strata from moisture and oxygen. However, these measures are only partially effective in supporting rock strata. In recent years, longwall mining techniques brought about the use of roof chocks and roof shields. These devices are self-advancing hydraulically to hold the roof in the immediate area of the longwall mining machine away from the machine as well as the operators therefor.
In recent years, roof bolting has become widely accepted. The roof bolts are effective to suspend the lower rock strata from upper competent strata. Similarly, other concepts utilizing the roof itself as a structural member are possible. In my prior U.S. Pat. Nos. 4,091,628 and 4,146,349, mine roof supports and rib supports are disclosed using an elastic member. The member takes the form of a curved plate that is prestressed by a flattening force against a surface of the mine opening. Various different forms of support are used for emplaced support of the plate. These include roof bolts inclined at an angle of 45.degree. to the plane of the plate.
Part of the rationale for selecting the size and strength of a roof support system is based upon the experience of roof falls in actual coal mining. A study shows that the median roof fall was only one-foot thick, and 90% of major roof falls involve roof strata four-feet thick or less. A median roof fall can be prevented by a 150-pound vertical force on each square foot of roof strata; also, to prevent major roof fall, a 600-pound vertical force on each square foot of roof strata is needed.
When, in situ, stresses exist in the upper roof strata of a mine opening, reactive forces to these stresses and tensile stresses from an external member such as roof bolts create a clockwise force couple at the left side of the mine roof strata and a counterclockwise force couple at the right side of the mine roof strata. The resultant forces are usually undesirable and adverse to effecting support through a beam action. It is a common practice as disclosed, for example, in U.S. Pat. No. 3,427,811, to incline roof bolts at an angle of 45.degree. for installation of truss supports. An analysis of this configuration of roof support indicates that force components are established at the point where the inclined bolt projects downwardly from the roof strata. These force components bend around the rock corner such that the stressed bolt imparts a concentrated compressive stress upon the immediate roof strata. This is undesirable because it tends to cause buckling of the lower roof stratum. When the anchoring roof bolt is fully grouted and resin-anchored as is the case with many such roof bolts, the effective locus of the anchor may be at the corner where the bolt extension bends around the lower stratum and/or around a spacer block near the corner. Except for the corner bearing on the bolt, the point anchoring at the upper end of the bolt produces a resultant force in the strata in a direction of 62-1/2.degree. from the horizontal. In other words, when roof bolts penetrate the roof strata at an angle of 45.degree. toward the side rib, the resulting force is oppositely directed at an angle of 62-1/2.degree. from the horizontal. This upward-point loading is a suspension effect applicable to the local region not a beam effect induced in the native roof strata from rib-to-rib.
The present invention provides a method and apparatus to control the counterclockwise and clockwise force couples in the immediate roof strata so that these forces are imposed at anchoring sites on the general strata at a distance away from the immediate native roof strata whereby undesirable force components do not enter the local beam support function. The present invention is directed to a systemic beam support that includes utilization of native roof strata as a major component of the systemic beam. In contrast to this, the current practice of anchoring trusses with roof bolts inclined at a 45.degree. angle does not produce a beam-type support but rather only a suspension-type support of localized regions of mine roof. The present invention is based on the discovery that by using roof bolts or other anchoring devices to impart the prestressing tensile force as a lower element of the beam from an anchor point at a considerable distance into the strata above the seam at some slight angle of between 0.degree. and 30.degree. from the horizontal, the tensile element is incorporated as part of a systemic beam support system. The most desirable angle is that which is provided by a true catenary curve at the point of attachment or at the point of entry into the anchoring hole. Avoiding a discrete angular change at this transition point avoids the concentration of stresses at this region. Due to force couples and the inaccessibility of the upper strata, there is no effective way for imparting a compressive stress into the upper layers of the strata at, for example, 3-6 feet above the roof. However, by anchoring the roof bolts or other members on the general strata, the reactive compressive stress to which these members are subjected is distributed to the general rock measures.